This invention relates to radiation sensitive devices and more particularly, but not exclusively, is concerned with radiation sensitive plates for the production of lithographic printing plates.
Such radiation sensitive devices are known and comprise a substrate, e.g. a metallic sheet, coated with a radiation sensitive layer. In use of such devices in lithographic printing plate production, the radiation sensitive layer is exposed to radiation using a transparency so that parts of the layer are struck by the radiation and other parts are not. In the case of negative-working radiation sensitive layers, the radiation struck areas become less soluble than the non-radiation struck areas. In the case of positive-working radiation sensitive layers, the radiation struck areas become more soluble than the non-radiation struck areas. Thus, by treating the image-wise exposed layer with a developer liquid for the more soluble areas, these areas can be selectively removed from the substrate to form an image constituted by the less soluble areas. This image constitutes the printing image of the eventual printing plate and the non-printing areas of the plate are constituted by the surface of the substrate revealed on development.
The printing image and the non-printing areas are essentially co-planar and the lithographic printing process depends upon the differing affinities of the printing image and the non-printing areas for ink and water. The printing image is ink-receptive and water-repellant and the non-printing areas are water-receptive and ink-repellant. During printing, water is applied to the plate and is received by the non-printing areas and repelled by the printing image. Then, ink is applied and this is repelled by the wet non-printing areas and received by the printing image. The ink is then transferred from the printing image onto the paper or the like to be printed.
When image-wise exposing a radiation-sensitive plate in the production of a lithographic printing plate it is essential that there is good contact between the transparency through which the plate is to be exposed and the radiation sensitive layer of the plate itself. The contact is achieved by using a printing down frame in which the plate and transparency are positioned between a flexible backing member and a glass sheet. The air between the backing member and the glass sheet is evacuated causing the plate and transparency to be squeezed together. This process is conventionally referred to as vacuum drawdown.
However, it is possible for pockets of air to be trapped between the smooth surface of the radiation sensitive layer of the plate and the transparency preventing, or at least extending the time required to achieve, the necessary contact. To overcome this problem, the radiation sensitive layer may be given a roughened surface which provides channels through which such air pockets can be evacuated.
There have been many suggestions as to how the roughened surface may be provided and in this regard reference may be made to UK Patent Specifications No. 1495361, No. 1512080, No. 2046461, and No. 2075702 and European Patent Specification No. 21428.
Japanese Patent Specification No. 98505/76 discloses spraying a waxy or fine-powdered resin from a solvent liquid onto the surface of the radiation sensitive layer so as to leave sprayed particles on the surface.
UK Patent Specification No. 2043285 discloses spraying the radiation sensitive layer with a powder and UK Patent Specification No. 2081919 discloses spraying the radiation sensitive layer with a watersoluble resin from an aqueous solution.
Whilst these suggestions all improve the vacuum drawdown they have certain disadvantages such as lack of adhesion of the sprayed particles to the radiation sensitive layer or incompatibility of the sprayed material with the radiation sensitive layer or with the developer liquids commonly used to develop the radiation sensitive layer after image-wise exposure.
To overcome these disadvantages, European Patent Specification No. 174588 discloses providing the surface of the radiation sensitive layer with a covering layer having the same composition as the radiation sensitive layer by spraying the radiation sensitive layer with a solution containing the same components as the radiation sensitive layer. Whilst this approach gives an improvement it still has certain disadvantages which are detailed in WO 87/03706 wherein the radiation sensitive layer is sprayed with a discontinuous covering layer which is more light sensitive than the radiation sensitive layer.
According to the present invention, there is provided a method of producing a radiation sensitive device which comprises
(i) providing a substrate carrying a radiation sensitive layer, PA1 (ii) providing a solution or dispersion, in a liquid hydrocarbon, of a material which is soluble or dispersible in developer for the radiation sensitive layer, and PA1 (iii) directing said solution or dispersion towards the radiation sensitive layer by a spraying technique to form a discontinuous covering layer of the material on the surface of the radiation sensitive layer. PA1 i) whether the material is to be sprayed as a solution or as a dispersion, PA1 ii) the nature of the radiation sensitive layer onto whose surface the discontinuous layer is to be applied, and PA1 iii) the composition of the developer to be used for the radiation sensitive layer PA1 i) the spray liquid is unlikely to dissolve or disrupt the surface of the radiation sensitive layer, PA1 ii) the spray liquid can be applied to the surface of a wide range of radiation sensitive layers, and PA1 iii) the solution or dispersion can, because of its low conductivity, be sprayed using the technique of electrostatic atomisation disclosed in EP A-0344985.
The solution or dispersion may be applied by a conventional spraying technique wherein the liquid is disrupted into drops (atomised) by means of mechanical forces. These can be turbulent air, mechanical shear (rotating disc/bell) or expansion of the liquid as it is pumped at high pressure through a small orifice (airless spraying). Conventional electrostatic spraying, which also uses one of these methods to atomise the liquid to be sprayed can be used. In this case the atomised droplets produced are electrostatically charged to attract them to the grounded substrate in order to improve deposition efficiency.
An alternative technique which can be used is the electrostatic atomisation technique described in our EP-A-0344985. In this technique, the solution or dispersion preferably has a conductivity of from 10.sup.3 to 10.sup.9 pSm.sup.-1 and a potential of at least 5 kV relative to the substrate is directly or indirectly induced in the solution or dispersion so that it forms drops in the absence of any other disruptive forces acting on it. This has the advantage that drops of more uniform size are produced.
In use of this electrostatic atomisation technique, the liquid is drawn out into one or more ligaments which break up into substantially equal sized drops which are attracted to the surface of the radiation sensitive layer due to the potential difference. Because the drops are of substantially the same size, the evaporation of the hydrocarbon liquid can be controlled so that all the drops reach the surface of the radiation sensitive layer at a similar degree of wetness. Preferably, the potential applied or induced is from 5 (preferably 10) to 35 kV of either polarity relative to the substrate. Too low a potential for a given liquid feed rate can give insufficient force to properly atomise the liquid giving a wide variety of drop sizes. Too high a potential can cause corona discharge from the tips of the ligaments which also gives a wide variety of drop sizes. Typically, the liquid feed rate may be from 0.05 to 2.0 cm.sup.3 per min per ligament. The size of the drops produced can be varied by adjusting the parameters: liquid feed rate, liquid conductivity or potential applied Reducing the liquid flow, increasing the liquid conductivity or increasing the potential applied all reduce the drop size. Larger changes in drop size can be achieved by varying two or more of the parameters simultaneously.
Generally, the liquid hydrocarbon used in accordance with the present invention will have a boiling point of from about 150.degree. to 200.degree. C. Typically, the hydrocarbon has from 6 to 11 carbon atoms and examples of suitable hydrocarbons are aliphatic hydrocarbons such as those commercially available under the trade designations Isopar (Exxon Chemicals Ltd) and Shellsol T (Shell Chemicals Ltd) and aliphatic/aromatic hydrocarbon mixtures such as White Spirit 100 (Exxon).
The material of the discontinuous covering layer may or may not be radiation sensitive.
The nature of the material for the discontinuous layer is determined by:
If the radiation-sensitive coating is, for example, a naphthoquinone diazide sensitised cresol novolak resin which after exposure is developed using a dilute aqueous alkaline solution, the material of the discontinuous covering layer will ideally also be soluble or dispersible in the same dilute aqueous alkaline solution. The use of an organic solvent or mixture of solvents as developer would similarly and ideally require the use of a material for the discontinuous layer which is also soluble or dispersible in the same solvent or solvent mixture.
There is also a need for the material employed for the discontinuous covering layer to be relatively transparent to the region of the spectrum to which the underlying radiation sensitive layer is to be exposed. It also needs to be sufficiently tough to provide an adequately abrasion resistant discontinuous layer.
The material for the discontinuous layer can be applied in solution but is preferably applied as a dispersion in the hydrocarbon solvent. If a solution is employed then suitable materials for the discontinuous covering layer are, for example, Plioway EC, a vinyl acetate terpolymer supplied by the Goodyear Tire and Rubber Company, and a copolymer of a C.sub.4 -C.sub.10 alkyl ester of methacrylic acid (70-90 mole %) and methacrylic acid (10-30 mole %). In the event of a dispersion being used, the choice of suitable materials for the discontinuous covering layer becomes very wide subject to the additional requirement that it can be used to produce a stable dispersion in a hydrocarbon liquid. If necessary non-hydrocarbon solvents may be added to assist in dispersion or dissolution.
For environmental reasons there has been a gradual moving away from the use of organic solvents in the processing of radiation sensitive plates in lithographic printing plate production. As a result the bulk of both positive and negative plates employed in the printing industry at the present time require the use of an aqueous based, usually alkaline, developer liquid. In order to ensure that the material constituting the discontinuous layer for such plates are soluble/dispersible in such aqueous based developer liquids, groups conferring water/aqueous alkali solubility need to be incorporated within its structure. Examples of such groups are --OH, --COOH, --SO.sub.3 H, --PO.sub.3 H, --SO.sub.2 NH-- groups and their corresponding anions. Typical materials for the discontinuous covering layer are polymers prepared by copolymerizing one or more of acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid and fumaric acid with one or more of styrene, vinyl toluene, ethylene, propylene, vinyl acetate, methyl methacrylate, butylmethacrylate, ethyl acrylate and acrylonitrile. Water soluble polymers such as cellulose derivatives, poly vinyl alcohol and polyacrylic acid are also suitable. Organic solvent-developable printing plates ideally require that the material of the discontinuous layer be soluble or dispersible in the same organic solvent as is in the developer liquid. Many polymer types are suitable as such a material including epoxy resins, alkyl(meth)acrylate copolymers, styrene maleic acid ester copolymers, and novolak resins.
The material of the discontinuous covering layer will generally be tacky to facilitate its adhesion to the radiation sensitive layer. This can be achieved by incorporating adhesion promoting materials or low levels of high boiling point tackifying solvents Alternatively, or additionally, the desired tackiness can be achieved as a consequence of the technique used to form the dispersion of the material.
Preparation methods for suitable dispersions are described in `Dispersion Polymerisation in Organic Media` edited by K. E. J. Barrett. These include techniques such as dispersion polymerisation, non-aqueous aqueous emulsion polymerisation, molten polymer emulsification, resin solution emulsification, precipitation, flushing from aqueous emulsions and a variety of milling procedures. Non-aqueous polymerisation techniques are particularly useful in that they employ dispersants which cause the dispersed particles to have an intrinsic tackiness which can promote self-fixing to the surface of the underlying radiation sensitive layer. Additional adhesion promoting materials can be added as can low levels of high boiling point tackifying solvents.
The advantages of using a hydrocarbon liquid either as solvent or dispersion medium for the material forming the discontinuous layer include
The radiation sensitive layer on which the discontinuous covering layer is formed may be a positive-working material such as a composition comprising a novolak resin and a naphthoquinone diazide ester or a negative-working material such as a composition as described in our European Patent No. 0030 862.